Many reports have described degenerative changes in porcine bioprostheses implanted in either the mitral or aortic valve position. None, however, have described morphologic alterations in bioprostheses in both positions in the same patient. The latter situation, of course, allows one bioprosthesis to serve as a control for the other. This report summarizes clinicopathologic findings in 5 patients in whom porcine aortic bioprostheses had been implanted 18 to 107 months earlier in both mitral and aortic valve positions. Porcine bioprostheses implanted in both the mitral and aortic valve positions simultaneously in 5 patients aged 20 to 61 years (mean 45) were reexamined 18 to 107 months (mean 51) later. In 4 patients, the degenerative changes were distinctly more severe in the bioprostheses in the mitral than in the aortic valve position.